The shape of the dose-response curve for benzene-induced leukemia at exposures below 10 ppm is a key public policy and risk assessment issue involving potentially billions of dollars. During the previous grant period, biological samples were collected from 250 workers exposed to a wide range of benzene concentrations (0.2 to 50 ppm as a time-weighted average, with 88% of the workers being exposed to <10 ppm) and 140 matched controls. These biological samples will be analyzed using a biomarker approach to (a) shed light on the dose-response curve for benzene's hematotoxic and genotoxic effects between 0.2 and 50 ppm in air and its mechanisms of toxicity; and, (b) develop new biomarkers of exposure, early effect and susceptibility for benzene. Leukemia-specific chromosome aberrations will be measured by fluorescence in situ hybridization in the metaphase spreads of lymphocytes of all 390 study subjects and the dose-response relationship evaluated. Further, using a new technique called OctoChrome FISH the effects of benzene on all chromosomes will be investigated to determine if it has selective effects. In a sub-group of 53 subjects hematopoietic progenitor cells circulating in the blood were cultured in colony forming assays, allowing us to evaluate the dose-dependent effects of benzene on these target cells. The level of genetic damage in progenitor cells will also be compared with that found in lymphocytes to determine if the latter are good surrogates. More than 40 genetic polymorphisms and folate and vitamin B status have been determined in all study subjects. This data will be correlated with data on chromosome damage and hematotoxicity to determine the role these effect-modifiers play in susceptibility to benzene. Finally, emerging technologies, such as microarrays and proteomics, offer a significant opportunity to develop new biomarkers and provide key mechanistic information. The use of Affymetrix microarrays is proposed and will reveal changes in gene expression related to benzene exposure. Further, application of a Ciphergen ProteinChip mass spectrometer will allow for differentially expressed proteins and protein modifications related to benzene exposure to be identified and a protein profile for benzene exposure to be established in serum and lymphocytes. The overall goal is to develop new biomarkers of exposure and early effect for benzene and provide new insights into the mechanisms of benzene toxicity. [unreadable] [unreadable] [unreadable] [unreadable] [unreadable]